Diphenhydramine / guaifenesin Side Effects
Gastrointestinal side effects of diphenhydramine have been usually mild and included nausea and dry mouth. Gastrointestinal side effects of guaifenesin have included stomach upset and vomiting with higher than recommended doses. Doses recommended for expectoration have rarely been associated with gastrointestinal upset.[Ref]
Most commonly, diphenhydramine hypersensitivity has manifested itself in patients receiving systemic drug after being sensitized to it by topical application. Sensitization with systemic administration has also been reported.[Ref]
The CNS depressant effect of diphenhydramine parallels its plasma concentrations. The plasma concentration threshold for sedation is 30 to 42 ng/mL, and to cause mental impairment is 58 to 74 ng/mL. Patients should be warned against driving while taking diphenhydramine.
Dystonic reactions have been accompanied by dizziness, mental confusion, rigidity, lip and tongue protrusion, trismus, torticollis, and swallowing difficulties and generally resolve spontaneously. Toxic encephalopathy has been reported in a child with chicken pox treated generously with topical diphenhydramine.
Nervous system side effects of diphenhydramine have included depression with drowsiness and sedation in nearly all patients treated. Motor skills may be impaired. Dystonic reactions have been reported after single doses of diphenhydramine. Nervous system side effects of guaifenesin have occasionally included dizziness and headache.[Ref]
Ocular side effects of diphenhydramine have included blurred vision, diplopia, and dry eyes due to anticholinergic effects.[Ref]
Dermatologic side effects of guaifenesin have included rare reports of rash.[Ref]
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Some side effects may not be reported. You may report them to the FDA.